Progressive slope aging process



Jan. 5, 1965 TAKASHI MATSUDA PROGRESSIVE SLOPE AGING PROCESS Original Filed June 9, 1960 TIME HLQNBULS INVENTOR.

TAKASHI MATSUDA ATTORNEY United States Patent 3,164,497 PRGGRESSIVE SLOPE AGING PROCES Talrashi Matsuda, Los Angeles, Calif, assignor to North American Aviation, Inc. Continuation of application Ser. No. 34,973, June 9, 1960. This application Feb. 8, 1963, Ser. No. 257,312 10 Claims. (Cl. 148-142) This case is a continuation of my previously filed and and co-pending application Serial No. 34,973, filed June 9, 1960, now abandoned.

This invention concerns an accelerated heat treatment process calculated to produce increased hardness or strength in metals and alloys, while requiring less time than conventional aging methods.

Although the invention disclosed herein is applicable to any material possessing the necessary strength improving characteristics with respect to aging time and temperature variations, the invention will be described in connection with the heat treatment of precipitation hardenable stainless steels for the sake of clarity.

In accordance with widespread industrial practice, parts made from material such as referred to above are often initially formed before any heat treatment operations are accomplished on such parts. Thus, the material may be in 'a relatively soft or ductile condition as received from the mill. The formed part thereafter may be heat treated in various ways to improve its mechanical properties. An illustrative example of the procedures generally referred to above may consist of the specific steps of forming a part of austenitic steel, annealing it at a high temperature, thereafter cooling it in a sub-zero environment to cause martensitic transformation, and finally aging the part for a protracted period at an elevated temperature less than the annealing temperature but sufiiciently high to increase its hardness or strength and temper the part. It is the final heat treating step, sometimes known in the art as the age hardening treatment, with which the invention disclosed herein is primarily concerned.

In a hypothetical problem situation which may serve to illustrate the instant invention without in any sense limiting the scope of the inventive concept, a part fabricated from a precipitation hardenable steel having less than 0.10 percent carbon, about 15 percent chromium, 7 percent nickel and 3 percent molybdenum may be heat treated in the manner described above. This alloy is typically designated at PHl5-7Mo, and after annealing has tensile ultimate and yield strengths on the order of 150,000 psi. and 55,000 p.s.i., respectively, and a Rockwell C hardness R of 27 maximum. Following the subzero heat treatment step referred to above, the stated material when completely transformed to martensite has ultimate and yield strengths on the order of 184,000 psi. and 126,000 p.s.i., respectively. After aging the martensitic part at 950 F. continuously for one hour, the part achieves its maximum physical properties, termed the final properties. In the stated case, the final properties include an ultimate strength approaching 225,000 p.s.i., while the yield strength is about 210,000 p.s.i. and the R hardness is about 50. Continuous aging at a constant temperature such as 950 F. in the example stated above is commonly termed isothermal aging. The final properties of the material after heat treatment are of major concern, and usually the property of greatest importance among these is the yield strength since that is the limiting design factor in many structures. Thus, any possible method for increasing the yield strength would strongly influence the production of structures involving this design factor. Accordingly, a distinct need exists in the industry for means by which an improvement in the yield strengths of materials may be effected in this manner. 7

Among the possible ways in which the final properties of metals and alloys may be enhanced, one possible approach lies in the variation of aging temperatures and periods to produce varying results according to the time, temperature, strength and hardness curves applicable to the material under consideration. Normally, there is a separate time-strength curve for each temperature at which isothermal aging may occur. Thus, for example, if a specimen of PH15-7M0 steel otherwise processed in the manner described above is aged for eight hours at a constant temperature of 950 F., the final ultimate and yield tensile strengths will be on the order of 230,000

p.s.i. and 215,000 p.s.i., respectively. By extending the aging period at any given temperature to sixteen or twenty-four hours, a considerable improvement in the final physical properties of the material can normally be effected.

However, prolonged aging periods in excess of eight hours are not well adapted to modern mass production methods, particularly where part sizes are relatively large and furnace capacity is limited. Therefore, a distinct need exists in industry for new heat treatment techniques by means of which superior physical properties of metals and alloys may be achieved without unduly multiplying the production time, labor force and capital expenditures associated with conventional heat treatment methods.

Accordingly, it is a general object of the invention disclosed herein to provide an improved method for achieving superior physical properties in metallic material by heat treatment of such material in less time than that required to achieve equivalent properties by conventional heat treating methods.

Other objects and advantages will become apparent upon a close reading of the following detailed description of an illustrative embodiment of the inventive concept, reference being had to the accompanying drawing showing typical time-strength curves for material of the type upon which the inventive heat treatment method disclosed herein maybe successfully practiced.

With reference to the illustrative example described above, it maybe seen from the drawing that the final heattreat properties of the stated type of material depend in part upon the temperature and duration of the aging proc ess. Thus, for example, curve A shows the variation in strength S obtainable by aging a specimen of the material such as PHl5-7Mo alloy at a given temperature for any particular period of time. For the sake of illustration, curve A may be assumed to represent an isothermal aging process in which the specimen is continuously exposed to a furnace temperature of 950 F. Since the variation of yield strength with time or temperature change generally follows the pattern of the ultimate strength in the stated material, the axis of ordinates in the drawing may be considered to represent either strength category. It may be seen from curve A that the greatest increase in strength S occurs during the initial portion of the aging period, reaching a maximum at the curve peak, and thereafter the strength decreases further with each additional increment of time. An increase in strength is thus represented by a positive slope in the initial portionof curve A, and

a decrease in strength occurs during the negative slope of the curve.

Curves B and C are isothermal aging curves generally showing the variation in strength S obtainable by aging material such as PHl5-7Mo alloy at lower temperatures than that denoted by curve A. For the sake of illustration, curves B and C may be assumed to represent aging processes at temperature values of 900 F. and 850 B, respectively. Each of the curves B and C has a characteristic generally similar to curve A, showing that the strength enemas? of a specimen at each aging temperature increases most rapidly during the initial portion of the aging period, but at a gradually decreasing rate until a point of maximum strength is reached. At this point the slope becomes zero, and beyond this point, the slope is negative denoting a decrease in strength with each additional hour of aging. This negative portion of the curve denotes over-aging of the specimen and is avoided where the purpose of heat treatment is to increase the strength of a part. 7 I

Considering first the strength obtainable from isotherrnal aging at the various temperatures represented by curves A, Band C, it maybe seen that in a given period of time t such as an hour, aging at the higher temperature represented by curve A will result in a higher strength S than aging for the same period at a lower temperature value. This is illustrated by the intersection of a line representing time period t .with curves B and C at points 2 and 3, respectively, denoting values of strength S and S lower than the strength 5 obtained from curve A.

In accordance with widespread industrial practice, where only a limitcd'tirne is available torheat treatment in the fabrication of metallic parts and such treatment is for the purpose of improving part strength, aging is accomplished for short periods of time at relatively high temperature. 111 the case of PH15-7Mo, aging at 950 F. for one hour is common under the conditions stated. When superior physical properties are more important than timing in a given mass production problem, aging is normally accomplished over long periods at relatively lower temperatures, and in the illustrative examples may advantageously con' sist of sixteen hours at 850 F., roughlycor-responding to a high point on curve C in the drawing. Thus, it is common practice to select a particular temperature and a .iixe d duration for isothermal aging of metallic parts representing a compromise between the relative importance of physical properties in the finished article, considered with the need for reasonable limitations upon the time allocated to this fabrication step.

By a unique process of heat treatment which departs from the conventional isothermal techniques previously known to the art and described above, the inventive concept disclosed herein permits achievement of superior physical properties such as associated, for example, with the peak of curve C in the drawing, by heat treatment of material in a relatively brief period of time such as associated with curve A. Thus, the invention disclosed herein obviates the need for compromise of the type referred to above. The inventive process is based upon a consideration of the isothermal aging characteristics of the material upon which the process may be practiced, with particular regard to the rates which strength of the treated part increases at each value of temperature used for aging. Thus, in the illustrative example set forth above and in the drawing, it may be seen that'the slope of curve A is highest nearest the curve origin, and'd'ecreases at an increasing rate as the curve Zenith is approached. Simsilarly, the slope of curve B is greatest at the initial portion of the curve, later decreasing to zero at its zenith.

At point 1 it may be seen that the slope ofcurve A, denoted by tangent line '7, has decreased to a lesser value than the slope of curve B denoted by tangent line 5% at point 2. Since points 1 and 2; represent identical periods of aging time t it is clear that the strengths of the material 'would be increasing at a more rapid rate at the stated moment of time if aging were accomplished at the lower temperature associated with curve B than at the high temperature of curve A. correspondingly, at another given period of time t the rate of increase of strength is more rapidat point 6 on curve C than at point 5 on curve B, as shown by the relative slopes of tangent lines 11. and a 10, respectively. I

Since the relative temperatures associated with curves A, B and C are high, intermediate and low," in that order, it is basic to the inventive concept herein that a progressive lowering of the furnace temperature from I 7 i a high initial value to various lower values, permitting brief periods of aging time at each lower value, results in a substantially constant rate of strength increase at each temperature until a maximum strength is reached. Thus, for example, in the illustrative example described above, a specimen of PHlS-TMO may be initially aged for one hour at 950 F., at the conclusion of which a point on curve A substantially coinciding with point 1 will be reached. At this point, it may be seen that the strength of the specimen is still increasing, but the rate of such increase is decaying rapidly. If the furnace temperature is lowered to 900 F, the relationship of time to strength in the specimen will assumea characteristic gencrally simulating that shown by curve B, with the curve displaced upwardly. Upward displacement of thestated relationship in the specime'nresults from the fact that the a strength achieved in the specimen at the high initial aging temperature is not lost merely by decreasing the furnace temperature to a lower value. Thus, when aging is continued at the new lower value of furnace temperature, the strength of the specimen continues to increase, but at the higher rate associated with the low temperature. After aging for one hour at the new lower temperature,lthe rate of strength increase again decays rapidly in the manner illustrated graphically by tangent l6 and point 5 on curve B; At this time, the furnace temperature is again lowered to some value such as 850 F, and the strength again increases at a higher rate corresponding generally to the characteristic shown by tangent 11 at point 6 on curve C,

but with the curve displaced upwardly due to' the fact that the strength previously gained by the specimen is not lost when aging is continued at a lower temperature. When the rate of strength increase at the new lower value begins to decay, the furnace temperature may again be lowered, and the pr cess described above is repeated for as many times as may be expedient or beneficial in achieving the best possible final properties in the specimen.

The net etfect of the gradual lowering of furnace temperature in the treatment process described aboveproduces a final strengtheven higher than'that which results from any isothermal aging treatment at one continuous value or such temperature. Thus, for example, the final strength of the specimen heat treated in accordance with the inventive teachings contained herein will be higher than the highest point on curve C. Moreover, total aging time required to achievethis superior strength is considerably less than that involved inisothermal aging, such as graphically illustrated by curve C to achieve the maxivalue of strength at the curve zenith.

Further illustrative of the phenomena described above and associated with the inventive process disclosed herein, typical data relative'to a number of specimens of PHl5-.-7Mo alloy heat treated by conventional techniques and bythe novel process of this invention are shown in tabular form below. Table'l shows representative values of ultimate and yield strength for separate specimens of the stated material identified by letters A through G, each aged isothermally by conventional procedures at the var ous temperatures and for the duration specified. in contrast to the results from heat treating methods of the prior art as represented by Table l, typical data relative to'the inventive process disclosed herein is shown in Table Table l p (PRIOR ART) 7 Yield Ultimate. Aging Aging Specimen Limit, Limit, Temp, Time, p.s.1. p.s.i. 1". Hrs.

210, 000 225, 000 A 950 1 215, 000 229, 000 950 4 215, 000 230, 000 950 s 211, 000 228, 000 900 4 218, 000 234, 000 900 s 209,000 I 229.000 850 s 212, 000 239, 000 s50 16 The data tabulated in Table I follows the typical pattern described herein and illustrated graphically by the drawing. By comparison of these data with Table II, it may be'seen that the novel process disclosed herein provides superior strength, both ultimate 'and yield, in an aging time considerably less than that required to ap-' proach an equivalent strength by conventional isothermal heat treatment. It is also apparent from data relative to specimens H and I that additional improvement using the inventive technique may be effected by variation of the precise temperatures and aging periods employed within a given total amount of aging time.

In further connection with the data shown by Table II, it is preferable in using the novel process disclosed herein that the specimen being heat treated remain in the furnace throughout the stated process, so that the specimen is furnace cooled to each lower temperature shown in the table rather than being allowed to cool to room temperature and then re-heated to the next lower temperature value shown. Thus, for example, it has been found expedient to lower the furnace temperature over a ten minute period from any given temperature value to the next lower value, so that the aging time at each period after the first is more accurately fifty minutes instead of one hour as shown for the sake of clarity on Table II. Alternatively, the novel process embodied in the inventive concept disclosed herein may advantageous ly be performed by initially heating a specimen to a predetermined high temperature and thereafter furnace cooling the specimen at a gradual uniform rate over a protracted aging period.

Since there is a general correlation between strength i and hardness, it follows that the improved strength of the heat treated specimen upon which the inventive concept disclosed herein may be practiced is accompanied by greater hardness. The stated correlation varies somewhat in accordance with several variables related to the precise composition of the treated alloy and the steps which may precede the novel heat treat process'described above, but the hardness of any given specimen markedly improves as a result of the novel process along a curve which generally reflects the improvement in strength ofthe specimen. However, as stated above, it is the yield strength which is normally of paramount importance to designers, and the improvement in -yield strength resulting from the novel process disclosed herein is readily apparent from Table II. Although particular values for a specific example of PH-7M0 alloy are set forth hereinabove, it will be understood by those skilled in the art that the same general type of alloy may have certain variations in the amount of its constituent elements without significantly altering the beneficial results of the inventive process disclosed herein. Illustratively, the novel process disclosed herein may be advantageously practiced upon specimens of substantially pure precipitation hardenable martensitic steel comprising from about .01 to ().10

'8 percent carbon, from about 10 to 20 percent chromium, from about 5 to 10 percent nickel, from about 2 to 4 percent molybdenum, and the balance substantially comprising iron. In addition to the alloys substantially set forth .hereinabove, the novel method taught herein may obviously be applied to any other materials having the necessary properties of isothermal aging suggested by curves A, B and C, for example, in the drawing.

. While the particular details set forth above and in the drawings are fully capable of attaining the objects and providing the advantages herein stated, the precise method thus disclosed is merely illustrative and could be varied or modified to producethe same results without departing from the scope of the basic inventive concept as defined in the appended claims.

I claim: V

1. A metal heat treatment process for hardening a metallic specimen capable of increasing its hardness by isothermal aging at a plurality of different temperatures wherein the amount of such increase at a first aging temperature varies in direct proportion to the duration of the aging period until a first peak value of hardness is reached, after which continued aging at said first aging tempera ture results in decreasing hardness, said specimen being i further capable of increasing its hardness by isothermal aging wherein the amount of such increase at a second aging temperature varies in direct proportion to the duration of the aging period until a second peak value of hardness is reached, after which continued aging at said said second aging temperature results in decreasing hard-s ness, said second aging temperature being less than said' first aging temperature and said second peak value being greater in amount of hardness than said first peak value, said second peak value requiring a longer aging period to achieve than the aging period required to achieve said first peak value, said process comprising the steps of:

heating said specimen in a furnace to said first'temperature, continually exposing said specimen to said first temperature for a period of time sufficient to obtain a substantial increase in the hardness of said specimen and no greater than the period at which maximum hardness of said specimen results from said temperature, thereafter lowering the heat in said furnace to said second temperature and continually maintaining said specimen at said second temperature for another predetermined period of time sufiicient to obtain a substantial increase in the hardness of said specimen and'no greater than the period at which maximum hardness of said specimen results from said second temperature, whereby superior physical properties of said specimen are produced in a total period of time less than that which would be required to produce equivalent properties by isothermal heat treatrality of different temperatures wherein the amount ofsuch increase at a first aging temperature varies in proportion to the duration of the aging period until a first peak value of hardness is reached, after which continued aging at said first aging temperature results in decreasing hardness, saidmaterial being further capable of increasing its hardness by isothermal aging wherein the amount of such increase at a second aging temperature varies in proportion to the duration of the aging period until a second peak value of hardness is reached, after which continued aging at said second aging temperature results in decreasing hardness, said material being further capable of increasing its hardness by isothermal raging wherein the amount of such increase at a third aging temperature varies in proportion to the duration of the aging period less than said second aging temperature, said second aging material.

unease? than said second peak value, said second peak value being greater in amount of hardness than said first peak value,

said third peak value requiring a longer raging period to achieve than the aging period required to achieve sa d second peak value, said second peak value requiring a longer aging period to achieve than the aging period required to achieve said first peak value, said process comprising the steps of: placing said material in'a furnace, heating said furnace to said first temperature, maintaining said first temperature for a first period of time sufficient to substantially increase the hardness of said-material, said first period of time being; not greater than the period required to achieve said'fi'rst peak value of", hardness, thereafter lowering said furnace heat to said second temperature, maintaining said second temperature for a second period of time sufficient to substantially increase the hardness of said material, said second period of time being not greater than the period required to achieve said second peak value of hardness, thereafter lowering said a furnace heat to said third temperature, and maintaining said third temperature for a third period of time sufiicient to substantially increase the hardness in said material, said third period of'time being not greater than the period required to achieve said third peak value of hardness, whereby superior physical properties of said material are produced.

'3. A process for heat treating a metallic alloy capable of being age-hardened to increase its strength at a vary ing but predictable rate when continuously exposed to a first temperature, said rate decreasing after a given period of time atsaid temperature, and therate of strength increase in said material when exposed to a second'and lower temperature being higher at said given period of time than said rate at said first temperature, theprocess comprising: exposing said material to said first temperature for said given period of time and thereafter exposing said material to said second temperature, so that the relatively high rate of strength increase in said material occurring during exposure to said first temperature is substar'rtially unchanged during exposure to-said second temperature. t t

i 4. The process set forth in claim 3 above, in which the change from said first temperature to said second temperature occurs rapidly and at a substantially uniform rate. I V

5. A process for heat treating metallic material capable of being age-hardened to increase itsstrength at a varying and different rate when aged isothernial-ly at a plurality of different age hardening temperatures, eachof said'ternperatures resulting in said strength increase until a maximum limit of strength is reached, said maximum limit being different for each of said temperatures, said maximum limit being followed by decrease of strength during continued isothermal aging at each of said temperatures, said process comprising: isotherrnally aging said, material at a contant elevated age hardeningYte-mperature for a period of time,-said temperature being below the melting point of said material but sufficiently high for 7 age hardening thereof, said period of time being sufficient to causea-suhstantial increase in hardness of said material, and thereaftersuccessively exposing said material to lesser values of age hardeningtemperature as determined by a comparison between'the length of'strength increase in said material occurring at each'value of age hardening temperature, so that a relatively high rate of strength intemperatures for a period of time equivalent to the total 7 sufi'icient to produce increased hardness in said steel, said period of time being no greater than the period at which maximum hardness of said steel results from said temperature, decreasing said first temperature rapidly to a second and lower furnace temperature sufiiciently high for age hardening of said steel, maintaining said second temperature for a second period'of time no greater than the period at which maximum hardness of said steel results from said second temperature, said scc'ond period of time being sufiicient to produce increase in hardness of said steel, decreasing said second temperature rapidly to a third and lower furnace temperature sufiiciently high for age hardening of said steel, and, maintaining said third temperature for a third period of time no greater than the period at which maximum hardness of said steel results from said third temperature, said third period of time being sufficient to produce increased hardmess in said steel, so thata higher yield strength is produced in said steel than that resulting from continuous exposure of said steel to; any ofrsaid first, second or third of said first, secondand third periods combined 7; A process forheat treating metallic material capable of being age-hardened comprising the steps of placing said material in a furnace, heating said material to a first temperature withinthe range from 925-975 F. for a period from /2 to 2 /2 hours, furnace cooling said material to a second temperature 20l00 F. less than said first temperature and continuously exposing said material to said second temperature for a period from g V to 3 /2 hours, furnace cooling said material to a third temperature Zil-IOO F. less than saids'econd temperature crease is obtained during the heat treatment of said. 7

a first temperature below the melting point; of said'steel' but sufilciently high for age hardening of sai'd steelurnain- I taining said'first temperature for a first 'period oftime V and continually exposing said material to said third temperature for a period from /2 to 3 /2 hours.

8. A method for precipitation hardening a stainless steel specimen comprising: heating said specimen to a first temperature ina furnace for aging at said first temperature for a first period of time, said first temperature being below the melting point of said specimenbut sufficiently high for age hardening of said steel, the duration of said first period depending upon the rate of increase of strength in said specimen at said first temperature compared withthe rate of strength increase in said specimen which would occur in the said first period at a second temperature lower than said first temperature, the second temperature alsobeing sufficiently high for age hardening of said steel, furnace cooling said specimen rapidly from said first temperature to said second temperature at the conclusion of said'first'period, and continuously aging said specimenat said second temperature for a second period, so that when the rate of strength increase occurring at the conclusion'of'said' first period is less than the rate of strength increase which would occur in said specimen'at an equivalent aging v its strength and hardnessduring' isothermal aging at various temperatures comprising: heating said material in a furnace to a high predetermined'value of temperature'suitable for age hardenin of said material, exposing said material to said temperature for a predetermined period of time'sufil'cient' to produce substantial increase in strength of said material', there'after progressively decreasing said'furnace temperature to a plurality of successively lower values, each of which is suitable for iso thermal age hardening of said material, and exposing said material to each of said lower temperature values for additional predetermined periods of time sufiicien't to produce substantial increase in strength of said material at. each of said lower temperature values individually, whereby the strength and hardness of said materialis increased at a rn'ean rate higher than the rate at which such increase would occur during isothermal aging of said material at any of said temperatures for a period of time equivalent to the total of said predetermined periods combined.

10. A process for heat treating a specimen of substantially pure precipitation hardenable martensitic steel comprising from about .01 to 0.10% carbon, from about 10 to 20% chromium, from about 5 to 10% nickel, from about 2 to 4% molybdenum, and the balance substantially comprising iron, said process comprising: heating said specimen to a first temperature within the range from 925 to 975 F. and aging said specimen at said first temperature for a first period of from one-half to two and one half hours, thereafter continuously cooling said specimen to a second temperature from 50 to 150 F. less than said first temperature and aging said specimen at said second temperature for a second period of from one-half to three and one-half hours, thereafter continuously cool- References Cited by the Examiner UNITED STATES PATENTS 2,048,164 7/36 Pilling et a1. 148142 2,519,406 8/50 Scott et a1. 148-:38 2,879,194 3/59 Eichelberg'er 148142 FOREIGN PATENTS 432,815 8/35 Great Britain. 439,804 12/35 Great Britain.

DAVID L. RECK, Primary Examiner. 

1. A METAL HEAT TREATMENT PROCESS FOR HARDENING A METALLIC SPECIMEN CAPABLE OF INCREASING ITS HARDNESS BY ISOTHERMAL AGING AT A PLURALITY OF DIFFERENT TEMPERATURES WHEREIN THE AMOUNT OF SUCH INCREASE AT A FIRST AGING TEMPERATURE VARIES IN DIRECT PROPORTION TO THE DURATION OF THE AGING PERIOD UNTIL A FIRST PEAK VALUE OF HARDNESS IS REACHED, AFTER WHICH CONTINUED AGING AT SAID FIRST AGING TEMPERATURE RESULTS IN DECREASING HARDNESS, SAID SPECIMEN BEING FURTHER CAPABLE OF INCREASING ITS HARDNESS BY ISOTHERMAL AGING WHEREIN THE AMOUNT OF SUCH INCREASE AT A SECOND AGING TEMPERATURE VARIES IN DIRECT PROPORTION TO THE DURATION OF THE AGING PERIOD UNTIL A SECOND PEAK VALUE OF HARDNESS IS REACHED, AFTER WHICH CONTINUED AGING AT SAID SAID SECOND AGING TEMPERATURE RESULTS IN DECREASING HARDNESS, SAID SECOND AGING TEMPERATURE BEING LESS THAN SAID FIRST AGING TEMPERATURE AND SAID SECOND PEAK VALUE BEING GREATER IN AMOUNT OF HARDNESS THAN SAID FIRST PEAK VALUE, SAID SECOND PEAK VALUE REQUIRING A LONGER AGING PERIOD TO ACHIEVE THAN THE AGING PERIOD REQUIRED TO ACHIEVE SAID FIRST PEAK VALUE, SAID PROCESS COMPRISING THE STEPS OF: HEATING SAID SPECIMEN IN A FURNACE TO SAID FIRST TEMPERATURE, CONTINUALLY EXPOSING SAID SPECIMEN TO SAID FIRST TEMPERATURE FOR A PERIOD OF TIME SUFFICIENT TO OBTAIN A SUBSTANTIAL INCREASE IN THE HARDNESS OF SAID SPECIMEN AND NO GREATER THAN THE PERIOD AT WHICH MAXIMUM HARDNESS OF SAID SPECIMEN RESULTS FROM SAID TEMPERATURE, THEREAFTER LOWERING THE HEAT IN SAID FURNACE TO SAID SECOND TEMPERATURE AND CONTINUALLY MAINTAINING SAID SPECIMEN AT SAID SECOND TEMPERATURE FOR ANOTHER PREDETERMINED PERIOD OF TIME SUFFICIENT TO OBTAIN A SUBSTANTIAL INCREASE IN THE HARDNESS OF SAID SPECIMEN AND NO GREATER THAN THE PERIOD AT WHICH MAXIMUM HARDNESS OF SAID SPECIMEN RESULTS FROM SAID SECOND TEMPERATURE, WHEREBY SUPERIOR PHYSICAL PROPERTIES OF SAID SPECIMEN ARE PRODUCED IN A TOTAL PERIOD OF TIME LESS THAN THAT WHICH WOULD BE REQUIRED TO PRODUCE EQUIVALENT PROPERTIES BY ISOTHERMAL HEAT TREATMENT OF SAID SPECIMEN AT EITHER SAID FIRST OR SAID SECOND AGING TEMPERATURE ALONE. 